Toulouse, chief town of the Tectosagi, at the end of the second century B.C. tried to shake off the yoke of Rome during the invasion of the Cimbri, but at the beginning of the empire it was a prosperous Roman civitas with famous schools in which the three brothers of the Emperor Constantine were pupils. In the fourth century it was reckoned the fifteenth town in importance in the empire.

In 413 it was taken by Astulph, the Goth, and in 419 under Wallia it became the capital of the Visigothic Kingdom. In 508 after conquest by Clovis it became Frankish. Legends of more or less recent date claim that it was evangelized by St. Martial, but as far as historical evidence goes the see seems to have been founded by St. Saturninus (Sernin) in the middle of the third century. The Passio Sancti Saturnini corroborates this date as that of his incumbency and martyrdom. Subsequent tradition claims that he was a disciple of St. Peter. St. Papoul was his companion and like him a martyr.

St. Honoratus, given in some lists as St. Saturninus's successor, is recognised as a pre-Schism Western saint by the Orthodox Church and it is therefore wrong to suggest that he seems just to have crept in through error from the fabulous legend of St. Firminus of Amiens.

To this epoch belongs a change that took place in the history of the Diocese of Toulouse. It decreased in size but increased in dignity. Before 1295 the Diocese of Toulouse was very extensive. At the beginning of the thirteenth century Bishop Fulk had wished to divide it into several dioceses. In 1295 a portion of territory was cut off by Boniface VIII to form the Diocese of Pamiers. Then in 1319 John XXII cut off the Diocese of Toulouse from the metropolitan church of Narbonne and made it a metropolitan with the Sees of Montauban, Saint-Papoul, Rieux, and Lombez as suffragans; a little later Lavaur and Mirepoix also became suffragans of Toulouse. The majority of these sees were composed of territory cut off from the ancient See of Toulouse itself.

The first archbishop was Raymond de Comminges, Bishop of Maguelonne from 1309, who, when created cardinal in 1327, abandoned the See of Toulouse and went to Avignon where he died in 1348. He left a book on the "Passion of the Saviour", and some "Sermons for Festival Days". Among his successors were: the Dominican William de Laudun (1327–45), previously bishop of Vienne; Raymond de Canilhac (1345–50), cardinal in 1350; Cardinal Francis de Gozie (1391–92); Bernard du Rosier (1451–74), author of two treatises on the temporal power of the pope and on the liberty of the Church, and who founded at Toulouse the College de Foix for the support of twenty-five poor scholars, where he collected one of the first libraries of the period; John of Orléans (1503–33), cardinal in 1533.

Protestantism entered Toulouse in 1532 through foreign students. As early as 1563 the Catholics of Toulouse founded a league to uphold the prerogatives of Catholicism, protected by the Parlement but jeopardized by certain Protestant town-councillors. From 1586 to 1595 the League party under Montmorency, Governor of Languedoc, and the Duc de Joyeuse held control in Toulouse. The rule of Henry IV of France was definitively recognized there in 1596. During this period of religious unrest Toulouse had many notable archbishops: Gabriel de Gramont (1533–34), cardinal in 1530; Odet de Châtillon, Cardinal de Coligny (1534–50), who became a Calvinist, married in 1564, and died in 1571; Anthony Sanguin (1550–59), Cardinal de Meudon in 1539; Georges d'Armagnac (1562–77), cardinal in 1544; François de Joyeuse (1584–1605), cardinal in 1583 and who conducted the negotiations between Henry IV and the Holy See.